It is well known how to compress ammonia gas by using a rotary compressor that is cooled and sealed by oil injection, and to separate the oil from the discharge gas in a tank from which the compressed gas is sent to its final destination whereas the oil, separated from the gas, is reinjected into the compressor for further sealing and cooling.
Hereafter, ammonia gas will be designated by xe2x80x9cammoniaxe2x80x9d whereas the condensed gas will be designated as xe2x80x9cliquid ammoniaxe2x80x9d
This oil, that catches most of the heat of compression, is often cooled by an injection of liquid ammonia into the compressor.
The amount of liquid injection is usually controlled by the discharge temperature of the gas to insure that the gas remains superheated and that no liquid ammonia reaches the oil tank.
Indeed, inside the oil tank, there are generally coalescing elements, made of fine plastic fibers of which the role is to capture the maximum amount of oil droplets in the discharge gas; and those fibers do not like to get in contact with liquid ammonia that would easily attack and destroy them. Coalescing elements, made of metal fibers or metal strips, are too coarse and do not offer enough contact surface to properly catch the oil droplets.
One important downside of that process and corresponding systems is that the compressed ammonia contains a sizable amount of oil in vapor form.
Indeed, to keep the gas superheated means that the oil has to be at a temperature above the ammonia condensing temperature by generally as much as 10 to 20xc2x0 C. and this leads to a substantial increase in the oil vapor pressure that can mean a few parts per million of weight of oil vapor in the gas, comparable to the weight of oil entrained as droplets.
For large coalescing elements, it is common to achieve an oil content in the 5 to 10 ppm (parts per million in weight of oil compared to the weight of compressed ammonia) range, and going below 2 to 5 ppm becomes nearly impossible with even very large coalescing fibers because of the oil temperature and the corresponding departure of oil in vapor form.
This invention relates to a process to produce nearly oil free compressed ammonia, including using a rotary compressor to compress ammonia, injecting a liquid into the compressor, discharging the compressed ammonia mixed with said liquid into a separator tank, separating said compressed ammonia from the liquid and reinjecting said liquid into the compressor, cooling the compressed ammonia to absorb at least partly the heat of compression and characterized by maintaining liquid ammonia, at least in droplets form, in at least part of the separator tank, through the amount of cooling.
This invention relates also to a system allowing to implement substantially the process described above, i.e. a system to produce nearly oil free compressed ammonia, comprising a rotary compressor, having at least one discharge pipe connected to a separator tank, at least one pipe connecting such separator tank to at least one injection port in said compressor, means to cool the compressed ammonia discharging from the compressor and characterized in that there are means to create droplets of liquid ammonia in the compressed ammonia stream, means to collect at least part of them and to return them to the compressor.
In a particular embodiment, this invention relates to a system to produce nearly oil free compressed ammonia, comprising a rotary compressor, having at least one discharge pipe connected to a separator tank, at least one pipe connecting such separator tank to at least one injection port in said compressor, means to cool the compressed ammonia discharging from the compressor, means to control the amount of such cooling, means to measure the liquid level in said separator and characterized in that the means to control the amount of cooling are actuated by the means to measure said liquid level.
The advantages of the process and its corresponding system is that, as there is liquid ammonia in at least part of the separator tank, the compressed ammonia is at ammonia condensing temperature, i.e. 10 to 20xc2x0 C. cooler than in conventional systems; and this creates a substantial decrease in the amount of oil entrained as vapor.
But unexpectedly, the total elimination of the coalescing element or their replacement by coarse elements in metal does not induce a higher carry-over of oil in droplet form.
On the contrary, injecting liquid ammonia and oil in approximately equal proportions and compressing ammonia at pressures corresponding to a condensing temperature of 45xc2x0 C., it has been found that the amount of oil departing with the compressed gas was below half of a ppm and could be nearly qualified as oil free compressed ammonia.
Without this being proven and just as a means of providing a possible explanation for this remarkable result, it is possible that the droplets of liquid ammonia, flying with the discharge gas or created in the separator tank, offer a huge surface of contact, equivalent to the surface of the fibers in a coalescing element, that can easily capture the flying droplets of oil and that the size of the ammonia droplets being larger than the droplets of oil because of the lower surface tension of the oil, they fall faster, hence more of the oil droplets can be collected before they leave the separator tank than if the oil droplets were left alone.
In the above invention, the rotary compressor can be any rotary compressor such as rotary vanes, rotary pistons, scrolls or screws.
The tank can be of any convenient shape such as vertical or horizontal, even though a horizontal shape is more appropriate as it gives more time for the droplets to be collected.
The oil can be any mineral oil or chemical compound having lubricating properties and not miscible with ammonia.
The liquid injected from the tank into the compressor can mean oil or a mixture of oil and liquid ammonia.
In a first embodiment of the invention, the cooling of the compressed ammonia coming out of the compressor is achieved by sending the mixture of gas and liquid through a heat exchanger, for instance a heat exchanger cooled by water. Some of the heat of compression remains in the compressed gas since its pressure and condensing temperature increased but the rest is removed by the water.
In a second embodiment of the process and system, the cooling of the compressed ammonia discharging from the compressor is done by an injection, into the compressor, of liquid ammonia coming from an auxiliary source such as a condenser where, in refrigeration systems, the compressed ammonia is condensed.
This liquid is compressed with the ammonia gas and flashes in the discharge pipe, absorbing at least part of the heat of compression.
But this invention would not be modified by combining the first and second embodiments and cooling through a combination of auxiliary liquid ammonia injection and of a heat exchanger.
In a third embodiment, the oil and liquid ammonia falling into the separator tank, are extracted separately from this tank and mixed in a substantially controlled percentage before being reinjected into the compressor.
As oil is a much better sealant than liquid ammonia but creates a much higher viscous drag, this allows to change the concentration of oil as a function of the speed of the compressor and to select the percentage that gives the best efficiency i.e. the best compromise between increasing volumetric efficiency by increasing the percentage of oil and reducing the friction power loss by increasing the percentage of liquid.